Generally speaking, a pipetting device which comprises one or more air-displacing piston stroke pipettes for use in laboratories serves to aspirate liquids from one container and dispense them into another container. The aspiration and/or dispensation of liquids can also take place in steps from or into different containers and with different liquids.
If liquids are frequently changed, it is useful if pipette tips that come into contact with the liquid to be transferred are designed so as to be replaceable and inexpensive, thus ensuring that carryovers and time-consuming rinsing steps during the liquid change are avoided.
As a rule, pipette tips are therefore injection-molded from a plastic material. The plastic materials used must be ultrapure, i.e., they should contain as few additives or plasticizers as possible to ensure that the transferred liquid is not contaminated, they should resist corrosion by chemicals and solvents commonly used in the laboratory, and they should be inexpensive.
It was found that a suitable plastic material for this purpose is polypropylene.
Pipetting devices were initially manufactured with only one channel and a manual drive. In this case, a pump system that is connected to the exchangeable pipette tip via an air channel (pipetting channel) is positioned inside the pipette.
The volume to be aspirated or dispensed is predetermined by the pump system and transferred to the liquid in the pipette tip via the air in the pipetting channel. In addition to the pump system, the quantity of air in the pipetting channel, the surface of the liquid level in the pipette tip and the cleanliness of the inlet and outlet aperture of the pipette tip, it is especially the hermetic seal between the pipetting channel and the pipette tip that plays an important role in ensuring the precision of the volume of aspirated or dispensed liquid.
This hermetic seal is typically implemented by creating a seal in the form of a cone between the pipette tip and the pipetting channel. In some systems, this seal is enhanced by an O-ring on the pipette cone. For manually operated pipettes, this principle offers the advantage that the pipette tip can be sealed off from the pipetting channel simply by sliding the pipette cone onto the pipetting channel cone. The tip can subsequently be just as easily released by using an ejector.
In addition to the single-channel pipetting device, multi-channel pipetting devices with a tandem arrangement of up to 16 pipetting channels have been developed.
In addition, pipetting devices with pipetting channels in two-dimensional configurations (8×12 and 16×24) have been designed, with the number and configuration of the pipetting channels being determined by the sample carriers that meanwhile had become widely accepted in practice, such as, inter alia, microtiter plates.
As the number of pipette tips that are to be simultaneously slid on and ejected increases, the cone principle described leads to an increasingly high degree of technical complexity.
Because of the given tolerances of the pipette tips in the area to be sealed, it is increasingly more difficult, as the number of pipetting channels increases, to hermetically mount all pipette tips. The manufacturing tolerances of the pipette tips must subsequently be compensated for by way of the elasticity of the pipette tips or by sealing means, such as O-rings on the cones, which is difficult because of the forces required, in particular in the case of 96- and 384-channel pipetting devices.
For this reason, a special sealing principle has been introduced for use with multi-channel pipetting devices, according to which the pipette tips are pushed with their front end against an elastic sealing plate that lies close to a flexurally stiff flat plate of the pipetting device into which the pipetting channels extend. The pipette tips are released simply by canceling the pressure. Effectively designing the front end of the pipette tips with an appropriately matched sealing surface of the sealing plate will make it possible readily to seal as many as 384 pipette tips at the same time as long as the sealing surface if protected against surface damage and contaminations.
However, this sealing principle again requires high contact pressure to ensure a simultaneous and effective seal for all pipette tips.
To introduce this contact pressure uniformly into all pipette tips that are arranged in a grid, and especially into those arranged inside the grid, the tips have a flange-mounted shoulder from which the pipette tips are suspended in a solid flexurally stiff plate called a magazine. To ensure that all pipette tips are uniformly sealed, only the height of the shoulder on the side of the pipette tips is important, which is a parameter that presents no problem in injection molding technology.
To accelerate the change of the pipette tips, this magazine is not permanently connected to the pipetting device but forms an integral unit with the pipette tips that is detachable from the pipette device.
The magazine loaded with pipette tips is inserted into a suitable holding fixture, hereinafter referred to as magazine holder, in the pipetting device and, to create the seal, is pushed or pulled against the aforementioned sealing plate.
Due to the forces thereby required, all components located in the force transmission path must have a very stable design.